From: Peter Zijlstra
Sent: November 11, 2018 at 2:43:27 PM GMT
> To: Nadav Amit <na...@vmware.com>
> Cc: Ingo Molnar <mi...@redhat.com>, linux-kernel@vger.kernel.org,
> x...@kernel.org, H. Peter Anvin <h...@zytor.com>, Thomas Gleixner
> <t...@linutronix.de>, Borislav Petkov <b...@alien8.de>, Dave Hansen
> <dave.han...@linux.intel.com>, Kees Cook <keesc...@chromium.org>, Dave Hansen
> <dave.han...@intel.com>
> Subject: Re: [PATCH v4 05/10] x86/alternative: initializing temporary mm for
> patching
>
>
>
> I don't seem to have gotten patches 0-2,7 for some reason; I'll try and
> dig them out of the LKML folder.
>
> On Sat, Nov 10, 2018 at 03:17:27PM -0800, Nadav Amit wrote:
>> +void __init poking_init(void)
>> +{
>> + spinlock_t *ptl;
>> + pte_t *ptep;
>> +
>> + poking_mm = copy_init_mm();
>> + if (!poking_mm) {
>> + pr_err("x86/mm: error setting a separate poking address space");
>> + return;
>> + }
>> +
>> + /*
>> + * Randomize the poking address, but make sure that the following page
>> + * will be mapped at the same PMD. We need 2 pages, so find space for 3,
>> + * and adjust the address if the PMD ends after the first one.
>> + */
>> + poking_addr = TASK_UNMAPPED_BASE +
>> + (kaslr_get_random_long("Poking") & PAGE_MASK) %
>> + (TASK_SIZE - TASK_UNMAPPED_BASE - 3 * PAGE_SIZE);
>> +
>> + if (((poking_addr + PAGE_SIZE) & ~PMD_MASK) == 0)
>> + poking_addr += PAGE_SIZE;
>> +
>> + /*
>> + * We need to trigger the allocation of the page-tables that will be
>> + * needed for poking now. Later, poking may be performed in an atomic
>> + * section, which might cause allocation to fail.
>> + */
>> + ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
>> + if (!WARN_ON(!ptep))
>> + pte_unmap_unlock(ptep, ptl);
>> +}
>
> The difference in how we deal with -ENOMEM here is weird. I think we
> have a _lot_ of code that simply hard assumes we don't fail memory alloc
> on init.
>
> I for instance would not mind to simply remove both branches and let the
> kernel crash and burn if we ever fail here.
Actually, now that we removed the fallback of patching without poking_mm, a
failure to allocate poking_mm should have had a BUG_ON().
For the second case, I think we still need either WARN_ON() or BUG_ON(), at
least as some sort of an in-code comment. I’ll change it to BUG_ON() if you
prefer.
